Biology 9.2 Structure of DNA
DNA’s Structure
DNA’s winding staircase

By the early 1950s, most
scientists were convinced that
genes were made of DNA.
They hoped the mystery of
heredity could be solved by
understanding the structure of
DNA.

Watson and Crick, two
researchers at Cambridge
University, determined that a
DNA molecule is a double
helix; two strands twisted
around each other like a
winding staircase.
DNA’s winding staircase
 Each
strand of DNA
is made of linked
nucleotides; the
subunits that make
up DNA.
DNA’s winding staircase


Each nucleotide has three parts;
 a phosphate group,
 a five carbon sugar molecule,
 and a nitrogen containing base.
The nucleotides link by a weak hydrogen bond.
DNA’s winding staircase

The five-carbon sugar molecule in the backbone is called
deoxyribose.

Deoxyribose is the D in DNA.

DNA’s full name is deoxyribosenucleic acid
DNA’s winding staircase

While the sugar molecule and phosphate group that
make up the backbone spirals are the same for
each nucleotide molecule, the nitrogen bases will
be one of four types.
DNA’s winding staircase
•The four nitrogen bases
may be one of the
following
•Adenine
•Guanine
•Cytosine
•Thymine
DNA’s winding staircase



In 1949, Erwin Chagraff, a
biochemist at Colombia
University, made a
discovery.
She discovered that the
amount of adenine always
equaled the amount of
thymine.
Also, the amount of guanine
always equaled the amount
of cytosine.
DNA’s winding staircase


In 1952, the first highquality photographs of
DNA, high quality X-ray
diffraction photographs,
revealed images of DNA’s
structure.
Soon after this , Watson
and Crick used the
information from these
first images to create their
first model of DNA’s spiral
form.
DNA’s winding staircase



Watson and Crick
discovered that a purine on
one strand of DNA is always
paired with a pyrimide on
the opposite strand.
We call these pairings basepairs.
Each rung of the ladder of
the staircase design is made
up of one of these basepairs.
DNA’s winding staircase




Watson and Crick
discovered as well that
these pairings of the four
nitrogen bases are always in
specific base-pairs.
Adenine always pairs with
thymine.
Guanine always pairs with
cytosine.
We call these two groupings
complementary base-pairs
DNA’s winding staircase


In charts showing DNA’s
form, these complementary
base-pairings always show
the groupings of adeninethymine and cytosineguanine.
These will often be
displayed as base-pairs
abbreviated as
 A-T
 T-A
 C-G
 G-C
DNA’s winding staircase
Computer Lab:




After completing your worksheet:
today’s research assignment is to
use the internet to find, research
and write a short report
describing the following IN
YOUR OWN WORDS.
1: Name and describe the three
parts of the DNA nucleotide
2: Demonstrate (by drawing a
chart of the DNA model) the
complementary base-pairing rules
3: Explain why the two strands of
the double-helix are called
complementary